Prior art circuits of this kind are known, for example, from German utility model 92 04 811, PAPST-MOTOREN, published Sep. 16, 1993, the entire con,tents of which are incorporated by reference, for the sake of brevity. Such devices are well adapted for driving combustion blowers, as taught by German patent application DE 41 41 106 A1, AMRHEIN et al./PAPST-MOTOREN, pub. 17 June '93, and which are driven by a collectorless dc motor.
Such devices must satisfy a number of requirements. Among other things, the rotary speed of the blower must be varied as a function of the desired heat output, in order to obtain optimal combustion of the fuel (oil or gas), and to minimize formation of toxic compounds in the exhaust gas. For such rotary-speed-regulated operation, dc motors are particularly well adapted, especially those operated in bridge circuits, since this permits a broad control range and simple control interventions. This is especially true of so-called two-pulse collectorless dc motors, as shown in U.S. Pat. No. 3,873,897, Mueller/Papst-Motoren, and German DE 23 46 380 C3. If such motors are constructed in single-strand (single-phase) form, this results in a very simple circuit and a simple motor structure, since only a single stator winding must be included, and the motor can thus be made compact. However, it should be noted that the present invention can also be used in multi-strand motors, e.g. for three-stranded (three-phase) motors which have three-pulse or six-pulse operating schemes.
The terminology used herein is more fully explained in the literature reference "ASR-Digest for Applied Drive Technology," by Rolf Mueller, 1977, pages 27-31. "Two-pulse" means that two current pulses are fed to the stator winding of the motor per 360.degree. el. rotor rotation, as in the case of the motor shown in FIG. 1 below. In a three-pulse motor, three current pulses are fed to the stator winding per 360.degree. el. rotor rotation, etc. "Single-strand" means that the motor has only one phase (strand), e.g. as shown in FIG. 1, following. "Two-strand" means that the motor has two phases, etc. To define an electronically commutated motor, both the number of pulses and the number of phases must be specified. In full-bridge circuits with transistors, one branch of the circuit employs pnp transistors, and the other branch employs npn transistors. While there are npn transistors whose voltage tolerance extends to 400 V, the highest voltage tolerance of pnp transistors is usually around 300 V, or maximally 350 V.
It is possible to drive such motors via MOSFET transistors with a rectified current derived from 230 V alternating current, e.g. as disclosed in EP 0 467 085 A1, HANS & MOINI/PAPST-MOTOREN, but not with a transistor full-bridge circuit, although this would be desirable on grounds of cost. The reason is that 230 V AC, with 10% overvoltage (which frequently occurs in power networks) upon rectification results, on the rectified-voltage side of the circuit, in voltage spikes of 358 V, and pnp transistors which stand up to such high voltage are not commercially available today.